1. Technical Field
This invention relates to the art of machining engine housings and, more particularly, to machining valve seats and valve stem guides in a deep-walled engine monoblock workpiece.
2. Description of the Prior Art
From the earliest use of internal combustion chambers having valve openings, engine housings have been split to contain valve seats about such valve openings in a head, thereby providing easy access for closely supported turning tools to machine or grind such valve seats. Hand-driven machine tools were the earliest used incorporating a pilot rod adapted to fit snugly within the valve stem bore as a concentric aligning guide for the machining of the valve seats (see U.S. Pat. Nos. 1,682,314; 1,721,310; and 2,977,727). A problem encountered with such method was that of misalignment of the pilot rod and the axis of the rotatable support for the cutting tool. This was solved by the incorporation of various types of flexible joints between the milling axis and the pilot tool with which the cutting tool was coupled (see U.S. Pat. Nos. 3,728,940; 4,147,462; and 4,545,706).
Such art does not address problems which have arisen with the recent advent of deep-walled engine monoblock workpieces. Monoblock workpieces with short cylinder walls were first used, such as shown in U.S. Pat. No. 3,521,613, to avoid these problems. This invention addresses directly such problems of machining valve seats associated with very deep-walled monoblock constructions, such as required for an iron monoblock insert housing shown in FIG. 1, which insert is cast into an outer unitary housing portion. When attempting to machine such a monoblock construction, it is difficult to support the machining tool because of the extreme axial distance the machining tool spindle must extend to reach the valve seats. Such difficulty of holding the spindle in close tolerance at the valve seat is exaggerated during high-speed rotary milling operations because of the inherent ability of the spindle to flex at such unsupported end. Even the use of a pilot rod extending into the pre-cast valve seat guide bore does not provide sufficient closely coupled support for maintaining the high degree of accuracy needed in such machining operations today. In fact, it is desirable to eliminate the valve stem guide support rod to enable the simultaneous machining of both the valve stem guide bore as well as the valve seat.